Several methods have already been proposed which employ the ultrasonic way in order to measure the distance separating two faces of an object. If the distance separating the two faces is substantial, there is in general no difficulty in interpreting the double signal received in the form of an echo since the faces generate two peaks which are clearly separated in time and which permit, knowing the speed of propagation of the wave in the medium in which it is propagated, calculation of the distance which separates the faces in question. However, when the faces are very close to one another and spaced a distance on the order of the wave length from one another, the echoes which they produce may appear merged.
Coating thicknesses applied to a substrate which thicknesses comprise between 5 and 20 .mu.m, have been apt to be measured by ultrasonic interferometry employing a frequency varying from 90 to 510 MHz. Such a method is described in the article entitled "Measurement of the Thickness of Thin Layers by Ultrasonic Interferometry" appearing in J. Appl. Phys. 55 (1), January, 1984, pages 194-198. However, this method furnishes a satisfactory result only if it is applied to a stable object, the dimensions of which are not subject to variations during the measurement.
The article entitled "Digital Signal Processing Method for Multilayered Media Thickness Measurement" which appeared in IEEE Ultrasonic Symposium 1988, pages 1027-1029, describes an ultrasonic measurement carried out on thin layers of paint applied in the automobile industry. In this case the layers exhibit thicknesses between 20 and 110 .mu.m. The central frequency here is around 75 MHz with a band width of 100 MHz. The measurement method is based on the analysis of a power cepstrum, defined as being the inverse Fourier transformation of the logarithm of the power spectral density of the radio frequency signal. Here however one is in the presence of superficial layers which permit working at a high frequency from whence derives a result enabling good resolution. This method is not suitable for the measurement of the thickness of an object situated within a medium attenuating the ultrasonic wave and which would thus oblige working at lower frequencies (10 MHz) as will be the case in a possible employment of this invention.
The cepstrum method is likewise the object of a thesis presented Feb. 9, 1979 at the National Polytechnical Institute of Grenoble by J. C. Balluet, which thesis is entitled "Les operateurs cepstres, applications a la separation d'echos rapproches". In this publication it will be noted however that the method employed for echoes which are very close to one another gives results which are difficult to interpret and that the observed peaks are located at the limit of visibility.
An ultrasonic echography method applied in vivo on living tissues is described in the article entitled "Range Resolution Improvement by a Fast Deconvolution Method", which appeared in Ultrasonic Imaging 6, 1984, pages 435-451. Numerous problems of mathematical physics may be formulated by means of an equation of convolution of the form y=h*x, where x and y denote a system of input and output respectively and where h is the pulse response of the system. The deconvolution is the inverse problem which consists in reconstructing the input on the basis of an output measured experimentally. Thanks to the method set forth in this article, it has been possible to study with precision the bottom of the eye and to follow the variations of the thickness of an arterial wall during the cardiac cycle. The method proposed however presents the disadvantage of separating only with great difficulty echoes which are very close in time.
Non-invasive sensors enabling the measurement of the interior diameter of an artery are known from U.S. Pat. No. 4,370,985 granted to Takeichi on Feb. 1, 1983. The indicated method gives no solution for measuring the actual thickness of the artery.